Optical system for optical disk, optical head unit for optical disk, and optical drive device

Abstract

An optical system for an optical disk, comprising a light source (61), a collimating optical system (63) for converting a luminous flux from the light source into parallel rays of light, and an object lens (66) for condensing the parallel rays onto the information recording surface (71) of an optical disk. The object lens consisting of two or three lenses and having a numerical aperture (NA) of at least 0.82 enables a high-density recording. Since a third-order astigmatism produced when the object lens is tilted 0.7 degree with respect to an optical axis is as small as up to 10 m lambda, a residual aberration, after a third-order comatic aberration produced when the optical disk is tilted due to warping or the like is corrected by tilting the optical lens, can be reduced. Therefore, a tilted optical disk still can ensure a satisfactory recording and/or reproducing quality.

What is claimed is:  
     
       1. An optical system for use with an optical disk, for recording and/or reproducing information to and/or from an optical disk, the optical system comprising: 
       a light source;  
       a collimating optical system for converting a luminous flux from the light source into a collimated light; and  
       an objective lens for focusing the collimated light on an information recording surface of the optical disk,  
       wherein  
       the objective lens has a numerical aperture (NA) of not less than 0.82,  
       the objective lens is composed of two or three lenses, and  
       a third-order astigmatism generated in the case where the objective lens is tilted 0.7 degree relative to an optical axis is not more than 10 mλ.  
     
     
       2. The optical system according to  claim 1 , wherein 
       the optical disk includes a transparent substrate with a thickness of approximately 0.1 mm on an objective lens side of the information recording surface, and  
       a third-order astigmatism, generated in the case where a third-order coma generated when the optical disk tilts by 0.7 degree is corrected by tilting the objective lens, is not more than 10 mλ.  
     
     
       3. The optical system according to  claim 1 , wherein a third-order spherical aberration that occurs when the luminous flux is focused via a transparent substrate with a thickness of 0.1 mm is not more than 70 ml. 
     
     
       4. The optical system according to  claim 1 , wherein a chromatic aberration of the objective lens at a wavelength of the light source is not more than 0.1 mm/nm. 
     
     
       5. The optical system according to  claim 1 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side, and  
       the first lens and the second lens are cemented to each other so as to compose a doublet.  
     
     
       6. The optical system according to  claim 5 , wherein the third lens has a flat surface on an optical disk side. 
     
     
       7. The optical system according to  claim 5 , wherein a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature. 
     
     
       8. The optical system according to  claim 5 , wherein the third lens has an aspherical surface on a light source side. 
     
     
       9. The optical system according to  claim 5 , wherein 
       a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature, and  
       a relationship expressed as below is satisfied:  
       
         
           −20<53 d+ 16 R− 69 f< 20  
         
       
        where R represents the radius of curvature of the surfaces of the first lens, d represents a thickness of the third lens, and f represents a focal distance of the objective lens.  
     
     
       10. The optical system according to  claim 1 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side,  
       the first lens and the second lens are cemented to each other so as to compose a doublet,  
       the first lens has a refractive index of not more than 1.54, and a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature of not more than 3.3 mm,  
       the second lens has a refractive index of not less than 1.7, and  
       the third lens has a refractive index of not less than 1.68 and a thickness of not less than 1.85 mm, and has an aspherical surface on the light source side and a flat surface on an optical disk side.  
     
     
       11. The optical system according to  claim 1 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side,  
       the first lens and the second lens are cemented to each other so as to compose a doublet,  
       the first lens has a refractive index of not more than 1.54, and a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature of not more than 3.1 mm,  
       the second lens has a refractive index of not less than 1.7, and  
       the third lens has a refractive index of not less than 1.75 and a thickness of not less than 1.85 mm, and has an aspherical surface on the light source side and a flat surface on an optical disk side.  
     
     
       12. The optical system according to  claim 1 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side,  
       the first lens and the second lens are cemented to each other so as to compose a doublet,  
       the first lens has a refractive index of not more than 1.5, and a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature of not more than 2.6 mm,  
       the second lens has a refractive index of not less than 1.74, and an optical-disk-side surface of the second lens has a radius of curvature of not less than 2.6 mm, and  
       the third lens has a refractive index of not less than 1.77 and a thickness of not less than 1.45 mm, and has an aspherical surface on the light source side and a flat surface on an optical disk side, the aspherical surface having a radius of central curvature of not less than 0.9 mm and not more than 1.4 mm.  
     
     
       13. The optical system according to  claim 1 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side,  
       the first lens and the second lens are cemented to each other so as to compose a doublet,  
       the first lens has a refractive index of not less than 1.4 and not more than 1.5, and a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature of not less than 2.3 mm and not more than 2.6 mm,  
       the second lens has a refractive index of not less than 1.74 and not more than 1.85, and an optical-disk-side surface of the second lens has a radius of curvature of not less than 2.6 mm, and  
       the third lens has a refractive index of not less than 1.77 and not more than 1.85, and a thickness of not less than 1.45 mm and not more than 1.8 mm, and has an aspherical surface on the light source side and a flat surface on an optical disk side, the aspherical surface having a radius of central curvature of not less than 0.9 mm and not more than 1.4 mm.  
     
     
       14. The optical system according to  claim 1 , wherein 
       the objective lens is composed of two lenses,  
       the two lenses are a first lens in a meniscus shape that is convex on a light source side, and a second lens in a planoconvex shape that is flat on an optical disk side and is convex on a light source side, which are arranged in the stated order from the light source side, and  
       both of the first lens and the second lens are made of resin materials.  
     
     
       15. The optical system according to  claim 1 , wherein a diffraction grating is formed on at least one surface of at least one lens among the lenses composing the objective lens. 
     
     
       16. The optical system according to  claim 2 , wherein a third-order astigmatism, generated in the case where a third-order coma generated when the optical disk tilts 0.7 degree is corrected by tilting the objective lens, is not more than 5.0 ml. 
     
     
       17. An optical head device for use with an optical disk, for recording and/or repro information to and/or from an optical disk, the optical head device comprising: 
       the optical system according to  claim 1 ; and  
       a mechanism for tilting the objective lens relative to the optical axis so as to correct a third-order coma generated due to tilting of the optical disk.  
     
     
       18. The optical head device according to  claim 17 , further comprising: 
       a luminous flux separating optical system for separating a luminous flux modulated at the information recording surface of the optical disk; and  
       a photodetector for receiving the luminous flux modulated at the information recording surface of the optical disk.  
     
     
       19. The optical drive device for recording and/or reproducing information to and/or from an optical disk, the optical drive device comprising the optical head device according to  claim 17 . 
     
     
       20. An optical system for use with an optical disk, for recording and/or reproducing information to and/or from an optical disk, the optical system comprising: 
       a light source;  
       a collimating optical system for converting a luminous flux from the light source into a collimated light; and  
       an objective lens for focusing the collimated light on an information recording surface of the optical disk,  
       wherein  
       the objective lens has a numerical aperture (NA) of not less than 0.82,  
       the objective lens is composed of two or three lenses,  
       the optical disk includes a transparent substrate with a thickness of approximately 0.1 mm on an objective lens side of the information recording surface, and  
       a third-order astigmatism generated in the case where a third-order coma generated in the case where the optical disk tilts 0.7 degree is corrected by tilting the objective lens is not more than 10 mλ.  
     
     
       21. The optical system according to  claim 20 , wherein a third-order spherical aberration that occurs when the luminous flux is focused via a transparent substrate with a thickness of 0.1 mm is not more than 70 ml. 
     
     
       22. The optical system according to  claim 20 , wherein a chromatic aberration of the objective lens at a wavelength of the light source is not more than 0.1 mm/nm. 
     
     
       23. The optical system according to  claim 20 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side, and  
       the first lens and the second lens are cemented to each other so as to compose a doublet.  
     
     
       24. The optical system according to  claim 23 , wherein the third lens has a flat surface on an optical disk side. 
     
     
       25. The optical system according to  claim 23 , wherein a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature. 
     
     
       26. The optical system according to  claim 23 , wherein the third lens has an aspherical surface on a light source side. 
     
     
       27. The optical system according to  claim 23 , wherein 
       a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature, and  
       a relationship expressed as below is satisfied:  
       
         
           −20<53 d+ 16 R− 69 f< 20  
         
       
        where R represents the radius of curvature of the surfaces of the first lens, d represents a thickness of the third lens, and f represents a focal distance of the objective lens.  
     
     
       28. The optical system according to  claim 20 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side,  
       the first lens and the second lens are cemented to each other so as to compose a doublet,  
       the first lens has a refractive index of not more than 1.54, and a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature of not more than 3.3 mm,  
       the second lens has a refractive index of not less than 1.7, and  
       the third lens has a refractive index of not less than 1.68 and a thickness of not less than 1.85 mm, and has an aspherical surface on the light source side and a flat surface on an optical disk side.  
     
     
       29. The optical system according to  claim 20 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side,  
       the first lens and the second lens are cemented to each other so as to compose a doublet,  
       the first lens has a refractive index of not more than 1.54, and a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature of not more than 3.1 mm,  
       the second lens has a refractive index of not less than 1.7, and  
       the third lens has a refractive index of not less than 1.75 and a thickness of not less than 1.85 mm, and has an aspherical surface on the light source side and a flat surface on an optical disk side.  
     
     
       30. The optical system according to  claim 20 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side,  
       the first lens and the second lens are cemented to each other so as to compose a doublet,  
       the first lens has a refractive index of not more than 1.5, and a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature of not more than 2.6 mm,  
       the second lens has a refractive index of not less than 1.74, and an optical-disk-side surface of the second lens has a radius of curvature of not less than 2.6 mm, and  
       the third lens has a refractive index of not less than 1.77 and a thickness of not less than 1.45 mm, and has an aspherical surface on the light source side and a flat surface on an optical disk side, the aspherical surface having a radius of central curvature of not less than 0.9 mm and not more than 1.4 mm.  
     
     
       31. The optical system according to  claim 20 , wherein 
       the objective lens is composed of three lenses,  
       the three lenses are a first lens that is a spherical lens, a second lens that is a spherical lens, and a third lens that is an aspherical lens, which are arranged in the stated order from a light source side,  
       the first lens and the second lens are cemented to each other so as to compose a doublet,  
       the first lens has a refractive index of not less than 1.4 and not more than 1.5, and a light-source-side surface and an optical-disk-side surface of the first lens have a same radius of curvature of not less than 2.3 mm and not more than 2.6 mm,  
       the second lens has a refractive index of not less than 1.74 and not more than 1.85, and an optical-disk-side surface of the second lens has a radius of curvature of not less than 2.6 mm, and  
       the third lens has a refractive index of not less than 1.77 and not more than 1.85, and a thickness of not less than 1.45 mm and not more than 1.8 mm, and has an aspherical surface on the light source side and a flat surface on an optical disk side, the aspherical surface having a radius of central curvature of not less than 0.9 mm and not more than 1.4 mm.  
     
     
       32. The optical system according to  claim 20 , wherein 
       the objective lens is composed of two lenses,  
       the two lenses are a first lens in a meniscus shape that is convex on a light source side, and a second lens in a planoconvex shape that is flat on an optical disk side and is convex on a light source side, which are arranged in the stated order from the light source side, and  
       both of the first lens and the second lens are made of resin materials.  
     
     
       33. The optical system according to  claim 20 , wherein a diffraction grating is formed on at least one surface of at least one lens among the lenses composing the objective lens. 
     
     
       34. The optical system according to  claim 20 , wherein a third-order astigmatism, generated in the case where a third-order coma generated when the optical disk tilts 0.7 degree is corrected by tilting the objective lens, is not more than 5.0 ml. 
     
     
       35. An optical head device for use with an optical disk, for recording and/or reproducing information to and/or from an optical disk the optical head device comprising: 
       the optical system according to  claim 20 ; and  
       a mechanism for tilting the objective lens relative to the optical axis so as to correct a third-order coma generated due to tilting of the optical disk.  
     
     
       36. The optical head device according to  claim 35 , further comprising: 
       a luminous flux separating optical system for separating a luminous flux modulated at the information recording surface of the optical disk; and  
       a photodetector for receiving the luminous flux modulated at the information recording surface of the optical disk.  
     
     
       37. The optical drive device for recording and/or reproducing information to and/or from an optical disk, the optical drive device comprising the optical head device according to  claim 35 .